Method and apparatus for vacuum coating metallic substrates



P. J. CLOUGH July 10, 1962 METHOD AND APPARATUS FOR VACUUM COATING METALLIC SUB STRATES Filed Aug. 13, 1958 22 OwN INVENTOR. BY Kid/ 4 J. mac/ 2s: s69 22 SN mm mm Ow ,Unitedi States This invention relates to coating and more particularly to high vacuum coating of a substrate, such as black iron plate, with a metal, such as aluminum. For convenience, the invention will be initially described in its use for the preparation of aluminum coated black iron which can serve as a substitute for tin plate. In the coating of black plate with aluminum, it ishighly desirable that the aluminum be deposited continuously on a black iron strip which is moved from the atmosphere through several stages of decreased pressure, through a coating chamber where a thin, shiny aluminum coating is applied by vaporizing the aluminumand condensing the aluminum on the black iron and then removing the strip through another series of seals back out to the atmosphere.

The present invention is particularly concerned with the problem of moving the substrate to the atmosphere after coating. In the past, many suggestions have been made for moving substrates into and out of a high vacuum coating chamber. Some of these suggestions have involved the use of a barometric leg for permitting passage of the substrate through a wide pressure differential without permitting atmospheric air to leak into the coating chamber. The present invention is particularly directed to an improved barometric leg construction which employs all of the advantages of previous barometric legs and additionally provides for positive maintenance of a high vacuum in the coating chamber which is free of vapors of the liquid forming the barometric leg.

Another object of the invention is to provide a high vacuum coating apparatus of the above type which can employ large clearances in the outlet seal so that the freshly-deposited aluminum coating will not be subject to scratching or abrasion.

Another object of the invention is to provide a coating apparatus wherein the outlet seal is made with sufficiently 11 atent ice ing chamber so that there is a slight flow of gases from the coating chamber into the trap chamber. In one preferred embodiment of the invention, the trap chamber is maintained at a pressure of about .15 micron Hg abs. Thus some gas will continuously flow from the coating chamber to the trap chamber through the seal between these two chambers. In the trap chamber, the'hot substrate passes into a barometric leg which isolates the trap chamber from the next succeeding chamber.

The liquid of the barometric seal is preferably a low vapor pressure oil such as a vacuum pump oil. Since the substrate is hot as it enters the oil, it will'tend to vaporize some of the oil and also raise the'temperature of the oil bath. Since these oil vapors would raise the total pressure in the trap chamber, it is necessary to condense these with a cooling means, such as a cooling coil for condensthrough several stages of decreasing pressure where the residual cleaning fluid (e.g. water) is evaporated and the substrate is preferably preheated to a temperature on the order of 400 F. Thereafter the substrate passes through a coating chamber which is maintained at a high vacuum on the order of 1 micron Hg abs. where the substrate is exposed to a stream of aluminum vapors which condenses on the substrate to form a shiny, adherent coating. The substrate then passesthrough a seal of rather wide clearance into a trap chamber which is preferably maintained at a pressure somewhat less than the pressure in the coating the vapors leaving the oil surface. A cooling coil is also preferably provided in the oil for maintaining the oil in the barometric leg at a predetermined low temperature to prevent undue heating thereof due to transfer of heat from the substrate to the oil.

Since it is desirable to maintain the trap chamber at an absolute pressure less than the pressure in the coating chamber, suflicient cooling surface is provided in the trap chamber to condense substantially all of the vapors emanating from the surface of oil in the barometric leg. Accordingly, the pressure of oil vapor in the trap chamber is maintained muchless than the total pressure in the coating chamber. Any residual vapors of oil in the trap chamber are prevented from diffusing into the coating chamber due to the fact that there is a countercurrent flow of gas from the coating chamber into the trap chamber through the seal connecting these two passages. In order to provide this countercurrent flow, one preferred embodiment of the invention has the high vacuum pump for evacuating the coating chamber connected to the trap chamber. Accordingly, the coating chamber is evacuated through the trap chamber thus inherently producing a lower pressure in the trap chamber than in the coating chamber.

In order to more fully understand the invention, reference should be had to the drawing, which is a diagrammatic, schematic representation of one preferred embodiment of the invention. In this drawing, the substrate is indicated at 10, the substrate being guidedthrough a coating chamber 12 where it is exposed to aluminum vapors from a coating source generally indicated at 14. As the substrate enters the coating apparatus, it passes from the atmosphere through sequentially arranged chambers 16,

18 and 20 ofdecreasing pressure. These chambers are separated from the atmosphere and each other by seals 22, 24, 26 and 28, these seals being diagrammatically illustrated as close fitting rolls with surfaces driven at the same speed as the speed of travel of the substrate. Ob-

viously, many other types of seals can be'employed, in-

*which is heated by an induction coil 32. The freshly than in the coating chamber 12.

' out of the coating apparatus through an intermediate pressure chamber 44 and a second barometric leg 46.

A cooling coil 50 is illustrated inthe trap chamber 36 above the surface of the liquid 40 forming the barometric leg. As mentioned previously, this cooling coil condenses all vapors generated when the hot substrate contacts the oil 40. It is also preferred; that a portion of this cooling coil be in contact with the main body of the liquid so as to keep the liquid 40. cool and prevent the heating from the coating chamber 12 to the trap chamber 36. It

thereof by the passage of the hotsubstrate into the liquid. Due to the excellent heat transfer between the oil and the substrate, the substrate will be cooled before it leaves barometric leg-4t). This oil can also serve as a protectivecoating for the aluminum coated black iron or the oil may be replaced by a different type of oil in the barometric leg 46 if such a different type of oil is desired. Obviously the requisite oillevels will bemaintained in the two barometriclegs by suitable additions of oil thereto.

Aplurality of vacuum pumps are preferably provided for maintaining the trap chamber at the lowest pressure in the pump 52. Suitable other vacuum pumps 54, 56, 58

and 60 are provided for evacuating chambers 44, 20, 18 and 16, respectively, tothe total pressures indicated for these various chambers.

In one preferred embodiment of the invention, the first chamber 16 .is maintained at about 100 mm. Hg abs.,

the second chamber 18 is maintained at about 1 mm. Hg

abs., the third chamber 20 is maintained at about 50 .microns Hgrabsand thelcoating chamber 12 is maintained at about 1 micron Hg abs. The trap chamber 36 is'rn'aintained at about .5 micron Hg abs, and the cooling coil is operated at a temperature onthe order of 5 C. to 10 C. This temperature is adequate to maintain the 'vapor pressureof a commercial, mechanical pump oil v40 at less than .5 micron Hg abs. As mentioned, the oil 40 is preferably a commercially available mechanical pump oil which can conveniently be a straight chain paraflin oil which has been treated to remove low boiling fractions. The oil 46 in the second barometric leg canbe the same as the oil in the first barometric leg 40 or can be a different oil depending upon the desired operating conditions. Equally, it can be another liquid such as water or the like, although an oil is preferred for the protection of the coated substrate.

The passage 34 through which the freshly-coated substrate passes from the coating chamber 12 to the trap chamber 36 is preferably many times thicker than the substrate so that there will be no danger of scratching the freshly-deposited aluminum coating. It is desired that this passage 34 have a length along the travel of the substratewhich is appreciably greater than the mean free path of residual gas molecules at thepressure of the system. This mean free path at 1 micron is about 2 /2 inches. With this construction, random diffusion of the vapors from trap chamber 36 into the coating chamber 12 will. not take place to any appreciable extent. When the-chamber 36 is maintained at a lower pressure than the coating chamber 12, there is a net flow of vapors desired, the passage 34 can have a height of several inches and can contain a number of plastic sheets extending generally transverse to the substrate in position to cut down random diffusion vapors from the chamber *36 to the chamber 12. In such case, it is preferred that the plastic sheets contact the freshly-coated aluminum surface only very lightly, if at all.

Numerous modifications of the invention may be made from the specific forms schematically illustrated and described above. For example, the coating chamber 12 may be provided with high vacuum pumps so as to maintain a pressure onthe order of 1 micron, and if desired, a slight leak of air or other gas can be provided to maintain the slight excess of pressure in the coating chamber 12 over the pressure in the trap chamber 36. The sheet material can be coated on both sides by providing a second source in the second chamber and passing the upper side of the substrate over the second source.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for coating a metallic substrate with another metal which comprises a high-vacuum coating chamber, means for maintaining the coating chamber at a pressure below about 10 microns Hg abs, means for passing the substrate into the coating chamber from the atmosphere, and means for removing the coated substrate from the coating chamber to the atmosphere, said removing means including a liquid barometric seal, a trap chamber between the coating chamber and the barometric seal, and means for maintaining the trap chamber atan absolute pressure less than that of the coating chamber.

2. Apparatusfor coating a metallic substrate with another metal which comprises a high-vacuum coating chamber, means for maintaining the coating chamber at a pressure below about 10 microns Hg abs., means for passing the substrate into the coating chamber from the atmosphere, and means for removing the coated substrate from the coating chamber to the atmosphere,- said removing means including a barometric seal, a trap chamber through which the substrate passes between the coating chamber and the barometric seal, and means for maintaining the trap chamber at an absolute pressure less than that of the coating chamber, said barometric seal comprising a liquid having a vapor pressure of 1 micron Hg abs. at a predetermined temperature, means for maintaining said liquid below said predetermined temperature despite addition of heat thereto by hot substrate passing into said liquid, and means positioned betweensaid coating chamber and said barometric seal for condensing vapors of said liquid at a temperature below said predetermined temperature.

3. The apparatus of claim 2 wherein said trap chamber and said coating chamber are connected by means of a passage having a thickness many times greater than the thickness of the substrate, said passage constituting the principal passage for vapors between said two chambers and said means for maintaining a lower pressure in said trap chamber providing a flow of vapors through said passage from the coating chamber to the trap chamber.

4. The process of coating a metallic substrate with another metal which comprises the steps of passing the substrate into a vacuum chamber maintained at a pressure below about 10 microns Hg abs, heating the substrate during its passage into said chamber, providing a stream of vapors of said other metal in said vacuum chamber, passing said metallic substrate through said stream of metal vapors, condensing vapors of said other metal on the hot substrate, passing the thus coated substrate into a second chamber, maintaining the second chamber at a pressure lower than the pressure in the coating chamber, immersing the hot coated substrate in a liquid bath in the second chamber to cool the substrate, condensing vapors of the liquid bath immediately adjacent the surface thereof, and passing the substrate through a barometric seal comprising said liquid.

1,595,486 Minton Aug. 10, 1926 6 Smith Apr. 30, 1929 McManus et a1 Aug. 14, 1945 Stoll Sept. 11, 1945 McManus et a1 Aug. 13, 1946 Novak Sept. 30, 1958 Schwindt Feb. 16, 1960 FOREIGN PATENTS Great Britain Aug. 2, 1923 Italy Mar. 26, 1964 Germany Oct. 25, 1956 

4. THE PROCESS OF COATING A METALLIC SUBSTRATE WITH ANOTHER METAL WHICH COMPRISES THE STEPS OF PASSING THE SUBSTRATE INTO A VACUUM CHAMBER MAINTAINED AT A PRESSURE BELOW ABOUT 10 MICRONS HG ABS., HEATING THE SUBSTRATE DURING ITS PASSAGE INTO SAID CHAMBER, PROVIDING A STREAM OF VAPORS OF SAID OTHER METAL IN SAID VACUUM CHAMBER, PASSSING SAID METALLIC SUBSTRATE THROUGH SAID STREAM OF METAL VAPORS, CONDENSING VAPORS OF SAID OTHER METAL ON THE HOT SUBSTRATE, PASSING THE THUS COATED SUBSTRATE INTO A SECOND CHAMBER, MAINTAINING THE SECOND CHAMBER AT A PRESSURE LOWER THAN THE PRESSURE IN THE COATING CHAMBER, IMMERSING THE HOT COATED SUBSTRATE IN A LIQUID BATH IN THE SECOND CHAMBER TO COOL THE SUBSTRATE, CONDENSING VAPORS OF THE LIQUID BATH IMMEDIATELY ADJACENT THE SURFACE THEREOF, AND PASSING THE SUBSTRATE THROUGH A BAROMETRIC SEAL COMPRISING SAID LIQUID. 